Social Comparisons and Groundwater Use

We provide a social comparison mailer to a randomly selected group of agricultural producers in an area of eastern Colorado overlying the High Plains Aquifer. The mailer graphically compares their 2018 groundwater use to other groundwater wells within their groundwater management district.

2019-05-01

2019-06-15

well-level groundwater use during the 2019 growing season

We randomly assign eligible agricultural producers in the study area of eastern Colorado to either a treatment or control group. Eligibility is based on ownership of one or more wells in the study area. The treatment group of producers received a mailer during the beginning of the 2019 growing season which compared their 2018 groundwater use to other groundwater wells within their groundwater management district (GWMD). The mailer graphically provided treated producers with information comparing the groundwater use of up to three of the wells that they own to well-level mean groundwater use and the water use of the 20th percentile well in the GWMD. The mailer also indicated the percentile of the GWMD groundwater use distribution each of their wells fell within, using the following language “Comparing your 2018 water use to other wells in the W-Y GWMD, your well(s) recorded use higher than X% of wells.”

Many agricultural producers in our study area operate multiple wells. The assignment to treatment or control groups occurred at the producer level, not the well level. Producers receiving the treatment mailers were provided with well-level comparison information, but due to size constraints on the mailer, recipients would see comparisons for a maximum of three wells. For producers that own or operate more than three wells, we limit the well-level comparison information to the three wells that utilized the most water in 2018. We also exclude wells that rank less than the 5th percentile among wells within their comparison group. Finally, some producers operate and own wells within multiple GWMDs. For these producers we determine their comparison group based on the GWMD the majority of their wells are located within.

To assign agricultural producers to treatment or control we follow a block randomization methodology wherein we block at the GWMD level and on past water use compared to their comparison group’s average. Specifically, we generate a variable that indicates whether a producer’s 2018 water use averaged across all their wells exceeded the median well-level water use within their comparison group. This blocking protocol resulted in 14 mutually-exclusive blocking groups. Assignment of producers to treatment or control was conducted using R’s randomizr package, which assigned 487 producers into the treatment group and 489 into the control group.

We randomize assignment using the R package randomizr

Individual (well owner / agricultural producer)

No

We did not utilize clustering in our methods

976 individuals (groundwater well owners / agricultural producers) which operate over 3,000 irrigation wells

487 individuals (groundwater well owners / agricultural producers) were placed in the treatment group, 489 were placed in the control group.

Given our sample design and stratification of assignment, our power analysis without covariate control reveals a minimum detectable effect size of 5%, with power = 0.8 and alpha = 0.05, to distinguish groundwater use differences between treatment and control wells. However, with the inclusion of covariate control in the power analysis, specifically groundwater management district specific effects lagged well-level pumping, we find a minimum detectable effects size of 0.03, with power = 0.8 and alpha = 0.05. We use data on past well-level pumping to conduct the power analysis.

Analysis Plan